With recent trends for luxurious interior of a vehicle, there have been an increasing number of cases where leather is used for large parts, such as a horn pad or an instrument panel equipped with an airbag therein, in addition to a leather-bound steering wheel or shift knob.
A plate-like base material for the horn pad or instrument panel is provided with slits as expected break lines. The slits allow the horn pad or instrument panel to immediately break and open when the airbag is inflated. The leather is attached to the base material with an adhesive so as to cover the front side (passenger side) of the base material, but should not prevent the horn pad or instrument panel (base material) from being broken and opened from the expected break lines.
Therefore, fragile portions need to be provided in the leather in accordance with the positions of the expected break lines in the base material. Note that, in order not to impair the sense of high quality created by the leather, the fragile portions need to be provided to be invisible to the driver or passengers of the vehicle. There has been adopted a method, for example, of making half-cut (cutting the leather while leaving a remnant of a predetermined thickness) slits (grooves), as the fragile portions, in the back side (surface to be attached to the base material) of the leather along with the expected break lines on the base material.
In the case of forming grooves in the leather, the thickness of the remnant of the leather in portions where grooves are made (portions where grooves are formed) (the remaining thickness of the leather in the groove portions) needs to have a proper and accurate value. If the remnant of the leather is too thick, the horn pad or instrument panel (base material) does not break from the expected break lines when the airbag is inflated. Thus, the leather may hinder the inflation of the airbag. On the other hand, if the remnant of the leather is too thin, lines (linear bulges or depressions of leather) indicating the positions of the grooves appear on the surface of the leather (on the side visible to a driver and the like), which impair the sense of high quality created by the leather.
To solve such a problem, there has been known an apparatus for forming a groove in leather (leather groove forming apparatus) in which a knife (blade) is attached to a head of an XY plotter (apparatus for moving a head in X-axis and Y-axis directions which are horizontal directions, by using an actuator such as a servo motor in accordance with a trajectory specified by a program). This technology is disclosed in, for example, Patent Documents 1 and 2 described below.